1. Field of the Invention
The present invention relates generally to PLL (Phase Locked Loop) circuits, and more particularly, to a PLL circuit for generating an output signal synchronized with an input signal, which is used for demodulation in, for example, a stereo receiver.
2. Background Art
Conventionally, as techniques for generating an output signal synchronized with an input signal, a PLL circuit has been generally used. Such a PLL circuit has been widely used as a stereo demodulator in, for example, a stereo receiver.
FIG. 1 is a schematic block diagram showing one example of such a conventional PLL circuit. In FIG. 1, the PLL circuit comprises an input terminal 1, a phase comparator 2, a low-pass filter (LPF) 3, a voltage controlled oscillator (VCO) 4, and an output terminal 5. In the PLL circuit shown in FIG. 1, the phases of an input signal inputted to the input terminal 1 and an output signal of the VCO 4 are compared with each other in the phase comparator 2. As a result, the phase comparator 2 outputs an error signal. This error signal is applied to the VCO 4 through the low-pass filter 3. An oscillating frequency of the VCO 4 is controlled by an output of this low-pass filter 3. An output of the VCO 4 is extracted through the output terminal 5. Such a conventional PLL circuit is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 7635/1982.
However, in the conventional PLL circuit shown in FIG. 1, the low-pass filter 3 is indispensable in order to remove an unnecessary component included in the output signal of the phase comparator 2 and apply to the VCO 4 only an error signal corresponding to the phase difference between the input signal and the output signal of the VCO 4. Therefore, if and when the above described PLL circuit is achieved as an integrated circuit (IC), a capacitor (not shown) constituting the low-pass filter 3 must be externally connected to the IC, so that the number of parts to be externally connected and the number of pins used for external connection are increased. Thus, integration is not improved and the manufacturing process is complicated, so that the cost is increased.